Abstract:

High junction temperature affects the optical performance and reliability of high power LEDs. This paper presents a study of the results of junction temperature measurement tests under various drive currents and ambient temperatures carried out on 3W white InGaN high power LEDs. The goal was to improve the accuracy of junction temperature characterization of the LEDs under test. Test results showed that 200 steps of pulse with 0.061% duty cycle can be used practically to construct reference plot of junction temperature (Tj) vs. forward voltage (Vf) at specific current levels for 3W high power LEDs so that Joule heating of LEDs can be ignored. High power LEDs mounted on an aluminum metal printed circuit board were tested inside a temperature chamber operated in temperature steps to reach a thermal equilibrium condition between the chamber and the LEDs. The LEDs were powered by 910μs pulsed currents with two different duty ratios (0.091% and 0.061%) between 0mA and 700mA. The diode forward voltages corresponding to the short pulsed currents were monitored to correlate junction temperatures with the forward voltage responses for calibration measurement. In junction temperature measurement, the junction temperatures at different constant currents and chamber temperature steps were estimated by in-situ monitoring of forward voltage responses. As a result, we could make realistic estimates of junction temperature to effectively control conditions for long-term aging tests of high power LEDs.

Index Terms — Light emitting diode, Reliability, Forward voltage, and Junction temperature

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